This invention relates to improved methods of photochemical dimerization of organic compounds, and more specifically for the hydrodimerization of simple and functionalized alkenes, the cross-dimerization of saturated compounds with simple and functionalized alkenes, and the dehydrodimerization of saturated compounds.
Mercury photosensitized dehydrodimerization has been known for many years. .sup.3 P.sub.1 Hg (=Hg*), formed by 254 nm irradiation of Hg vapor, attacks a variety of organic compounds to give dehydrodimers (e.g., the dehydrodimer bicyclohexyl can be obtained from two cyclohexane molecules with loss of hydrogen). See Steacie, E. W. R., Chem. Rev. 22, 311, 1938; Cvetanovic, R. J., Progress in Reaction Kinetics 2, pp. 39-77, 1964; Strausz, O. P., Gunning, H. E. JACS 95, 746, 1973. These dehydrodimers are much less volatile than the parent organic compounds and condense. In the condensed phase, they are protected from further attack, and in this way high yields of dehydrodimer can be obtained with high conversion. Only recently has this principle of vapor pressure selectivity been applied to mercury photosensitized dehydrodimerization. See U.S. Pat. No. 4,725,342 (Brown and Crabtree); Brown, S. H., Crabtree, R. H., J. Chem. Ed., 65, 290, 1988 and JACS, 1989 (in press). Quantum yields from 0.1-0.8 were obtained, but certain compounds with strong C--H bonds (CH.sub.4, C.sub.2 H.sub.6,(CH.sub.3).sub.3 COH) failed to give products. Such substrates are known to react very inefficiently with .sup.3 P.sub.1 Hg (see Cvetanovic, 1964, supra).
Many other mercury photosensitized processes have been described for treating organic compounds. U.S. Pat. No. 2,636,854 (Cier) discloses the production of aromatic compounds from alkanes. U.S. Pat. No. 2,640,023 (Cier) discloses the production of branched paraffinic hydrocarbons from a mixture of paraffinic hydrocarbons. U.S. Pat. No. 2,730,495 (Gray) discloses the production of alkyl and cycloalkyl hydroperoxides from a gaseous mixture of oxygen and one or more volatile paraffins or cycloparaffins. U.S. Pat. No. 2,908,622 (Bates) describes the production of formaldehyde from methane.
The mercury photosensitized reaction of hydrogen is a recognized source of hydrogen atoms. See Cario, G., Frank, J., Z. Physik, 11, 155, 1922. These hydrogen atoms can add to olefins to give free radicals which are known to give typical free radical reaction products, including hydrodimers. For example, the hydrodimer C.sub.4 H.sub.10 can be obtained from two ethylene molecules and a hydrogen molecule. See Cvetanovic, R. J., Advances in Photochemistry 1, pp. 115-82, 1963, and E. W. R. Steacie, Chem. Revs., 1938, 22, 311.
The mercury photosensitized reaction of nitrous oxide is a recognized source of oxygen atoms. These can add to unsaturated hydrocarbons or abstract H atoms from saturated hydrocarbons. See Cvetanovic, 1963, supra.
The present invention is the result of continuing investigations in this area in which the effect of carrying out mercury photosensitized reactions of functionalized alkenes in the presence of hydrogen was examined. Physico-chemical studies have involved the use of mercury photosensitization of H.sub.2 in the presence of simple lower alkenes, but in the sixty years that this has been known, it has not been considered for preparative work, let alone a candidate for the commercial production of dimers, for several reasons. The literature suggests that (1) fragmentation of the alkene-derived radical is an important side reaction, (2) hydrogenation of the alkene is an important, even a major reaction, (3) only small amounts of material can be treated, (4) expensive apparatus is required, (5) a mixture of products is obtained, and (6) functionalized products cannot be employed.
Concerning reasons (1) and (2), Steacie, the most highly regarded figure in the field, notes in his key review (E. W. R. Steacie, Chem. Revs., 1938, 22, 311) that Olsen and Meyers (J. Amer. Chem. Soc., 1926, 48, 389 and 1927, 49, 3131) observed hydrogenation of ethylene as the major pathway, fragmentation to methane as the next most important pathway, and dimerization as the least important pathway in the presence of excess H.sub.2. The only higher boiling products noted by H. S. Taylor and D. G. Hill (J. Amer. Chem. Soc., 1929, 51, 2922) were involatile teleomers of the form C.sub.n H.sub.2n, which were formed only in low yield. The only recent work in the area was carried out by Rabinovitch (I. Oref, D. Schuetzle, and B. S. Rabinovitch, J. Chem. Phys., 1971, 54, 575), who reported that Cis-2-butene was converted to butane, butene, and isooctanes of undetermined isomer distribution. The only alkenes of six carbons or more, e.g., 2,2,3-trimethylpent-1-ene, on the other hand, gave largely fragmentation to lower boiling species and did not give dimers (C. W. Larson and B. S. Rabinovitch, J. Chem. Phys., 1970, 52, 5181). Based on these reported investigations of mercury photosensitized reactions, one would not suspect that a specific isomer of a dimer could be obtained as the predominant reaction product, or that the reaction could be conducted on commercial scale, or that functionalized alkenes could be used, or that higher alkenes, e.g., with six carbons, or more could be dimerized. In fact, even the apparatus used in our own previous work (U.S. Pat. No. 4,725,342) is not satisfactory for the preparatory methods of this invention, which requires minimum residence time for the reaction products, is not highly selective.
H atoms addition to alkenes is known to produce "hot" (vibrationally excited) alkyl radicals. These are known to decompose more easily than "cold" radicals, and to react with H.sub.2 to give the corresponding alkane (B. S. Rabinovitch, S. G. Davis, C. A. Winkler, and Canad. J. Res., 1943, B21, 251). These radicals would be expected to give disproportionate alkane and isomerized alkene, which in turn would be expected to participate in subsequent reactions. Consequently, one would expect to obtain an undesirable mixture of products, even with simpler alkenes and especially so with functionalized alkenes.
U.S. Pat. No. 4,725,342 specifically states that alkenes do not dimerize under the conditions described therein. However, it has now been found that under certain conditions, rapid and efficient dimerization of simple and functionalized alkenes to give specific isomers of the dimer products is possible. Under those conditions, the H atoms selectively add to the alkene to give the more substituted free radical, and so the dimer of this radical is the major product of the reaction. ##STR1## EQU Hg*+H.sub.2 =2H.multidot. (2) EQU XCH=CH.sub.2 +H=X.dbd.CH--CH.sub.3 ( 3) EQU 2XDH--.dbd.CH.sub.3 =(CH.sub.3)XHC--CHX(CH.sub.3) (4)
The products can be similar to those obtained in the earlier dimerizations, reported by Brown and Crabtree, supra, but there are a number of significant advantages in this method of this invention. First, the selectivity of the process is determined by the alkene isomer employed, so that the product distribution can be in large measure predetermined. Instead of producing the mixture of all possible 2.degree.--2.degree. dehydrodimers which have formed from alkanes (2.degree.--2.degree. dimers are those that can be considered as being formed from the recombination of two 2.degree. radicals) formed from a compound containing a linear aliphatic chain, only a very restricted number of 2.degree.--2.degree. hydrodimers are formed from alkenes which structurally are capable of producing a mixture of hydrodimers. For example, essentially only 5,6-dimethyl decane is produced from 1-hexene. Not only is there an improvement in selectivity (and resultant yield of desired hydrodimer) but one can also use organic compounds as substrates for the reaction which are not effectively dimerized by methods described in the prior art. For example, Brown and Crabtree (J. Chem. Ed., 1988, supra) found that nitriles do not react and that alkenes, aldehydes and esters give complex product mixtures, and Cvetanovic (1963, supra) notes that perfluorinated compounds are essentially unreactive and that epoxides react to give a complex mixture of products. Therefore, an organic chemist would not suspect that the method would work for functionalized alkenes for the reasons (1)-(5) given above and for the following additional reasons: (7) the hydrogen atoms formed from H.sub.2 and Hg* should add to the unsaturated functional groups, rather than or as well as add to the C.dbd.C double bonds, (8) the functional groups should quench the Hg* and prevent it from breaking the H--H bond of H.sub.2, (9) functionalized alkenes, such as acrylonitrile or vinylsilanes, are particularly liable to polymerize, which Rochow observed for vinyl silanes under Hg photosensitization (D. G. White and E. G. Rochow, J. Amer. Chem. Soc., 1954, 76, 3897).
In the method of this invention, X in Formulae 3 and 4 can contain functional groups which would not be tolerated by our patented method ('342) and which would not be expected to survive H atom reactions. For example, esters give a large number of different products under the conditions of the '342 patent, but in accordance with this invention, unsaturated esters give high conversions and yields of the .alpha.--.alpha. dimers being those which can be considered as being formed from the recombination of two radicals in which the free radical center is .alpha. to the X group). A very significant example of the same pathway involves perfluorinated alkenes. The corresponding fluorocarbons are completely inactive under the conditions of the '342 patent, and no ways are know of dimerizing fluorocarbons efficiently. In contradistinction, perfluorinated alkenes are efficiently dimerized by the instant process.
Although a certain number of photochemical cross-dimerizations involving silanes and alkenes have been described (J. C. Dalton, Organic Photochemistry 1985, 7, 85), these are the only prior art examples of the general class of reaction between alkenes and saturated species described herein. For example, Haszeldine et al. (R. N. Haszeldine, S. Lythgoe, and P. J. Robinson, J. Chem. Soc. (B), 1970, 1634) found that Me.sub.3 SiH and F.sub.2 C.dbd.CF.sub.2 gave a variety of cross-dimers. It was suspected that Hg photosensitization was involved as a result of contamination of the reactor with Hg vapor. A mixture of products was obtained, such as Me.sub.3 SiCF.sub.2 H, Me.sub.3 SiCF.sub.2 CF.sub.2 CF.sub.2 CF.sub.2 H, and Me.sub.3 SiCF.sub.2 CF.sub.2 CF.sub.2 CF.sub.2 SiMe.sub.3. Based on this prior work in which several molecules of unsaturated component were usually incorporated into the products, it would not seem likely that one would be able to obtain clean formation of the simple cross-dimer incorporating one molecule of each component. Furthermore, a chain reaction was assumed (R. N. Haszeldine, M. J. Newlands, and J. B. Plumb, J. Chem. Soc. 1965, 2101). This erroneous assumption would lead one to predict that an isomeric structure for the cross-product opposite to the one found in accordance with this invention and opposite to the one found in genuine chain reactions. For example, Elad (D. Elad, Organic Photochemistry, 1969, 2, 168) found that 1-alkyl trioxanes were formed in his chain reactions involving 1-alkenes and trioxan, whereas 2-alkyltrioxan is obtained by the method of this invention.
Reactions of saturated hydrocarbons with hydrogen atoms has been described. However, instead of obtaining the hydrodimer obtained under the conditions of this invention, previous workers found that methane, formed by fragmentation of the free radicals was a major product. For example, methane formed 90% of the product from cyclopentane (H. I. Schiff and E. W. R. Steacie, Can. J. Chem., 1951, 29, 1), and methane was the only product from ethane (W. R. Trost and E. W. R. Steacie, J. Chem. Phys., 1948, 16, 361).
The dimerization of internal olefins over a promoted BF.sub.3 catalyst to yield oils useful as lubricant basestock is described in U.S. Pat. No. 4,319,064.